SOI technologies have been investigated extensively for a number of applications including radiation resistant or rad-hard devices, high-performance high-speed IC's, and 3-D integrated devices. Various techniques have been investigated for SOI material formation. These include:
1. Silicon-on-Sapphire (by epitaxial growth)
2. Silicon-on-CaF2 or BaF2 (by MBE)
3. SOI by selective epitaxial lateral overgrowth (ELO-SEG)
4. SIMOX (high-energy oxygen implantation into Si)
5. SOI by wafer bonding
6. Recrystallization techniques
Among those techniques, SOS, SIMOX, and SOI by wafer bonding are the most manufacturable techniques at present. In general, the SIMOX approach requires high-dose, high-energy O+ ion implantation. The optimization of the SIMOX material quality requires special attention to the implantation and subsequent annealing conditions. The single crystal silicon layer formed on the SIMOX wafer is rather than (e.g. less than a few thousand .ANG.). In some cases, it may be necessary to deposit a layer of epitaxial silicon on top of the SIMOX material in order to reduce the number of defects in the active device regions and fabricate devices away from the defective silicon/silicon dioxide interface.
The wafer bonding techniques employ various bonding methods such as annealing, mechanical pressing and annealing, or electric-field pressing and annealing. Various interlayer insulators such as thermal silicon dioxide and doped oxide have been used. Typical wafer bonding processes are based on the Van der Walls forces which require extremely flat and defect free surfaces. When the two silicon wafers are bonded together, various methods can be used to thin the wafers to form the SOI layers.
The SOI material fabrication by ELO-SEG (epitaxial lateral overgrowth-selective epitaxial growth) is based on selective epitaxial filling of seed holes and lateral epitaxial overgrowth followed by a subsequent etch back to produce thin epi on patterned oxide. The original oxide layer is patterned in order to provide silicon seed holes for SEG and subsequent lateral overgrowth. There is a trade-off between the epi material quality (and manufacturing cost) and the amount of electrical isolation between epi and semiconductor substrate.